Black hole noms: planetary treats for the galactic monster

Most galaxies – including ours – host a hungry monster. allthecolours

What do black holes eat? And do supermassive black holes have fiercer appetites? Let’s remind ourselves of the facts.

Lurking at the centre of the Milky Way is a monster, a giant black hole with a mass four million times that of the sun. With its immense gravitational pull, Galactic Central is a very dangerous place, with high energy radiation and stars zipping around several thousands of kilometres per second.

But the Milky Way is not alone in hosting a monster. It now appears most large galaxies possess these supermassive black holes at their hearts, some with a mass of more than a billion suns.

The largest black holes consume a lot of mass, and as infalling gas crashes together on its path to final destruction, it gets superheated and glows brightly. Emitting radio-waves to gamma-rays, these active galaxies are among the brightest beacons in the cosmos.

Comparatively, our Milky Way is more modest, and the central black hole is relatively less energetic. This is good for us, as, if properly fed, the outpouring of radiation from the central black hole could potentially destroy life on Earth.

Such a doomsday scenario was the basis of the science-fiction book Inferno by the late astrophysicist Fred Hoyle, and his son Geoffrey.

The book is based in fact, pointing out that in 3-4 billion years our black hole will burst into life during the collision between the Milky Way an its nearest large neighbour, the Andromeda Galaxy.

While quiet, our black hole is not silent, with irregular burps of infrared radiation and x-rays. Such energetic hiccups can only arise as the black hole is being fed, and being fed chunks rather than being drip-fed material. So what could be the source of the morsels and treats being consumed?

One recent suggestion from UK astronomer Kastytis Zubovas, based at the University of Leicester, is that the food for the flares are planets and asteroids.

These are not normal planets and asteroids, formed around stars in the depths of the Milky Way, that have wandered to close to the black hole. These are effectively recycled planets and asteroids.

x-ray delta one.

A planet like our Earth formed from a disk of debris that orbited our nascent Sun. Starting from small beginnings, it built up over time, from small grains of cosmic dust and rock colliding and sticking together.

In Zubovas’s picture, our central black hole is surrounded by an immense disk of debris, formed from pulverised stars and planets as they jostled in their orbits in the inner regions of the galaxy.

As the debris collided together, some begun to stick together and sink towards the black hole. Like a proto-planetary disk around the Sun, larger and larger bodies grew, forming planetary and astroid-sized objects.

But this is only a temporary rebirth from the ashes. As the planets sink closer to the black hole, the tidal forces from its gravity grow and grow, until, eventually, it is ripped apart again and swallowed into the hole, releasing a burst of energy in its death throes.

This gentle feeding of bite-sized pieces will continuously produce flares and flashes from the central regions of the galaxy at a low level.

Every hundred thousand years or so, the black hole will get a chance for a hearty meal, when an individual star makes its way down into the central regions to be torn apart. A vast amount of energy will be released and irradiate the surrounding regions, frying any nearby planets.

In fact, the leftover glow from such an outburst 300 years ago is still visible, imprinted on gas clouds near the Galactic Centre.

But we must remember that the monster at the heart of the Milky Way is only resting. It has a date with destiny, and the Andromeda Galaxy, in a few billion years time. Once the gas in that collision is funnelled down into the central black hole, there will be some serious galactic fireworks.

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